geometry.area_rectangle

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnly, idempotent, and closed-world, so safety is covered; the description adds genuinely new behavior: exact multiplication to the input's digit count, independent of the caller's Decimal precision. It does not say what happens on invalid or non-finite input (error vs. coercion), which is the remaining behavioral gap.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loads the formula and return type in the first sentence, then constraint, then precision semantics, then the misuse warning, then the square shortcut. No filler sentences; each one carries a distinct, actionable fact.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

An output schema exists, so return shape need not be explained, and the description covers inputs, semantics, and misuse cases adequately for a simple 2-parameter pure function. Only the invalid-input/error behavior is left unstated.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 0% — the properties only carry titles — so the description must carry the burden, and it does: both parameters are strings, must parse as finite non-negative Decimals, must share a unit, and are multiplied exactly. That is far more than the schema provides and fully compensates for the coverage gap.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource (compute rectangle area as width * height) and pins the numeric type to Decimal. Siblings like geometry.area_circle, area_triangle, and area_polygon are implicitly distinguished by the rectangle-specific formula and the square special case.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives explicit input conditions (finite, non-negative Decimal strings, matching units) and an explicit when-not-to-use warning that diagonal or perimeter values produce meaningless results. The square case is resolved with a concrete instruction to pass the same edge length twice.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.